CMOS transmission gate (T-gate) switches introduce distortion when used in an instrumentation amplifier (IA). A prior-art transmission gate circuit is shown in FIG. 1. Transistors MN1 and MP1 form the CMOS switch between pins “IN” and “OUT”. This switch is controlled by the “PGATE” and “NGATE” voltages, which are driven out of phase and rail to rail (0-5V) by two digital inverters.
The off-resistance of this switch is essentially infinite (>1000 meg). The on-resistance is typically between 50 and 5000 ohms, depending on design, and varies with process, supply voltage, and temperature (PVT). Furthermore, the on-resistance also varies with the voltage present at the “IN” and “OUT” terminals. The magnitude of this non-linear variation in resistance is typically significant, on the order of +/−10% of the nominal on-resistance over the full range of applied voltages in a given system. In amplifier circuits where low signal distortion is important, the effects of this non-linear on-resistance must always be mitigated.